Electrolytic reduction of nitric acid solutions containing radioactive waste



' ELECTROLYTIC; REDUCTION OF" NITRIC. ACID SOLUTIONS CONTAINING mono.v ACTIVE WASTE Henryi Ward Alter; :DanvillepCalifi, andDuane L. Barney,

Schenectady,= N Ya: assignors to. the; United. States of America ,as, representedbyrthe .United States; Atomic EnergyzCommissioin No Drawing; Application March '8, 19.57

Serial Nok- 644,958

7'Claims. (Cl. 23-.-159);'

This invention relates to a method ,forthe' treatrnentof nitric acidsolution s whichcontainradioactive fission products in a manner that will render the disposal of such radioactive materials both safe'and economical.

The invention is particularly adaptable ,to those. nitric acid 'solutions containing fission products: resulting from the processing of irradiated uranium fuel as obtained fromneutronic' reactors. The neutron bombardment of uraniumproduces valuable products'which ityis necessary to recover'from-the remaining uranium and fission productsr This is accomplished by either'of'two methods, one of which involves multiple. precipitations and the. other a combination of chemical precipitation coupled by Solvent extraction. The latter" method provides the .nitric acid solutionsused as' the starting materialin the present invention;

Disposal of these nitric acid solutions, particularly the radioactive fission productscontainedtherein, presents a verytroublesome problem. Direct storage has been suggested, however considerations of the volumes involved, cost of tank space andgeneral safety render such a plan unfeasible. It has also been suggested that the'volume be reduced by simple distillation. Such method has not proved successful because certain of the fission products are volatilized from an acid solution during distillation, serious corrosive conditions arise; and unavoidable organic contaminants react explosively with concentrated nitric acid. Another suggested method is to destroy the nitric acid with a reducing agent capable of reacting with the nitric acid to give gaseous products and then reducing the volume of the solution by evaporation. This method has not been used extensively because of health hazards and corrosive problems.

The principal object of the present invention is a method for the treatment of nitric acid waste solutions which will greatly reduce the volume of the radioactive materials to be disposed of. Other objects will be apparent to those skilled in the art upon reading the following description:

In general the process of the present invention involves neutralizing the nitric acid solution with an alkali such as an alkali metal hydroxide, removing the precipitate found and electrolyzing the resulting alkaline nitrate solution which results in the reduction of the nitrate to nitrite ion, ammonia, other gaseous products and converts the alkaline nitrate to hydroxide. The nitric acid solution forming the starting material for the process of the present invention contains about 95% of the fission products produced during the neutron bombardment of uranium and varying amounts of nitrate salts. It will range from 0.5 M' to 4"M in nitric acid and from 0.5 10- M-to 2 10- M in-uranium and 0.5 M to 6 M in total nitrates. Atypical analysis follows:

HNO M 1.9 Uranium M l.4 10' Total 3 activity c./m./ml 1.2 -10 Total gamma activit'y c./m.'/ml 5.5 l0

The complete process of the present invention contemplates three principal. steps, neutralization of the waste acid solution to yield an alkaline. nitrate solutionand. a.

precipitate of hydroxide, which is removed by filtration or otherwise, electrolysis of. the nitrate solutionto convert the. alkali nitrate, to alkali hydroxide, and to yield ammonia; and oxygem. which..incidentally maybe con-. verted to nitric acid by well'known catalytic methods,

and reduction; of the volume. of the hydroxide solution by evaporation. then be recycled to act asthebasefor neutralizing additional waste acid.

More specifically, the nitric -acidwaste is neutralized by'alkal'i hydroxide in an amount sufficient torender the A liter, the metal 'ion-of. which will precipitate as the hy-' droxidein alkaline solution, e. g. ferric nitrate, may be made before neutralization. The bulk of the fission products are removed from the solution alongwi'th the precipitatedhydroxides by hydroclone, centrifuge or byfil-tera-tion resulting-ma gross decontamination factor of 2-10. The. alkaline nitrate solution is then electrolyzed in-anelectrolytic cell, several types of which are available commercially. The gases produced during electrolysis are scrubbed by conventional means and may be released without hazard. However economies prescribe that the gaseous nitrogen products be retained for conversion to other useful products. The reduction of the nitrate results in the formation of an equivalent amount of alkali hydroxide. This basic solution, which may contain undestroyed nitrate and nitrite is evaporated to remove the water and any dissolved ammonia. The distillate which contains only a small amount of dissolved ammonia may be discarded. The concentrated alkaline hydroxide solution is then recycled to the neutralization step.

Accordingly, all scavengable fission products constituting most of the radioactive material, are removed in the precipitation step and since the volume of the precipitate is small, the space necessary for storage is greatly reduced. Non-scavenged fission products, such as ruthenium and cesiumwill' build up on successive recycle to produce a stream rich in specific fission products which may be recovered by ionic exchange or other means not involved in the method of the present invention. Fission products are not volatile in alkaline solution, therefore, adequate safety factors are obtained by mechanical deentrainment such as scrubbing. The fission products removed with the precipitate may be readily and safely stored indefinitely and may if desired be used as a radiation source.

The resulting hydroxide solution can 3 The invention is further illustrated by the example:

EXAMPLE I One thousand, liters of nitric acid waste solution from i the solvent extractionof uranium and plutonium was divided into equal parts. Ten g. of ferric nitrate was then added to one part and both parts were made about 0.25 M in sodium hydroxide. 'The resulting precipitate of hydroxides was removed by centrifugation. The supernatant solutions gave the following analysis.

-NaNOa concentration I ass 4.85 4. 5 4.85 NsOH concentration M.-- 0.26 '0. 26' I 0. 22 I 0. 23 NaNOz concentration M. 4X10 4X10-4 4X10- 4X10-4 Decontamination factors I I A decontamination of 400 represents approximately the lower limit tor the detection of Zr+Nh for the analytical method.

The solution was then subjected 'to electrolysis which was continued until the concentrationof the nitrate ion in the solution was'redu'ced to 0.25 M The solution'was y I in nitric acid, which comprises addlng, ferric ion as ferric nitrate in an amount sufficient to yield a solution approxithen evaporated to 25% of its original volume and the residue returned to theneutralization step. in theprocess.

What is claimed is: g l. A process for the treatment of a nitric acid solution. containing radio active. wastes, being substantially 0.5-4 Min nitric'acid, which comprisesneutralizing the solution with alkali hydroxide in an amount sufiicie'nt to make the solution 0.5-2 molar in alkali hydroxide and precipitate insoluble hydroxides, separating the precipitate, electrolyzing the solution to convert the alkali nitrate to alkali hydroxide, gaseous ammonia and oxygen, reducing the volume of the solution by evaporating the water and dissolved ammonia and recycling the concentrated solution.

following 1 2. f A process for the treatment of a nitric acid solution containing radio active wastes, being substantially 0.5-4 -M in nitric acid, which comprises neutralizing the solution with sodium hydroxide in an amount sufiicient to make the solution 0.5-2 molar in sodium hydroxide and precipitate insoluble hydroxides, separating the precipiacid solution containing radioactive wastes, which comand recycling the concentrated solution.

prises neutralizing the solutionwith sodium'hydroxide in an amount sufficient .to make the solution 1.2 molar in sodium hydroxide and precipitate insoluble hydroxides, Y separating the precipitate, electrolyzing the solution to" convert sodium nitrate to. sodium hydroxide, gaseous ammonia, and oxygen, continuing the electrolysis until the solution'is substantially 0.25 molar in nitrate'ion;

reducing-the ,volurneof the solution 75% by evaporation 4. The process of claim 1 in which .0l-1 gram per liter of ferric nitrate is added to the nitric acid solution prior to neutralization. I I

5. The process of claim 2 in which .01-1 gram per liter of ferric nitrate. isadded to the nitric acid solution prior to'neutralization. A

6. The'process of claim 3 In which .01--1 gram per liter of ferricnitrate is added to the nitric acid solution prior to neutralization. I I

7. A process for the treatment mately 8x 10" molar in ferric ion, neutralizing the solu nitrate to sodium hydroxide, gaseous ammonia and oxygen, reducing the volume of the solution by evaporating the water and dissolved ammonia and recycling the concentrated sodium hydroxide.

No references cited.

of amid acid solution containing radioactive wastesbeing substantially 1.9 molar, 

1. A PROCESS FOR THE TREATMENT OF A NITRIC ACID SOLUTION CONTAINING RADIO ACTIVE WASTES, BEING SUBSTANTIALLY 0.5-4 M IN NITRIC ACID, WHICH COMPRISES NEUTRALIZING THE SOLUTION WITH ALKALI HYDROXIDE IN AN AMOUNT SUFFICIENT TO MAKE THE SOLUTION 0.5-2 MOLAR IN ALKALI HYDROXIDE AND PRECIPITATE INSOLUBLE HYDROXIDES, SEPARATING THE PRECIPITATE, ELECTROLYZING THE SOLUTION TO CONVERT THE ALKALI NITRATE TO ALKALI HYDROXIDE, GASEOUS AMMONIA AND OXYGEN, REDUCING THE VOLUME OF THE SOLUTION BY EVAPORATING THE WATER AND DISSOLVED AMMONIA AND RECYCLING THE CONCENTRATED SOLUTION. 